Characterization of R-type vitamin B12-binding proteins by isoelectric focusing. II. Comparison of cobalophilin (r proteins) from different sources.

The R-type vitamin B12-binding proteins, here called cobalophilin, in human plasma, serum, leukocytes, gastric juice, amniotic fluid, and milk, were characterized by isoelectric focusing and gel filtration. Cobalophilin from all these sources in microheterogenous, consisting of several isoproteins with isoelectric points (pI) between 2.3 and 5.0. Isoproteins with pI values of 3.0, 3.3, 3.6, 3.9, and 4.2 were found in most of the cells and fluids studied, but in different proportions. Milk contains isoproteins with pI values mainly between 4.0 and 4.7, and such components also occur in considerable amounts in saliva. The cobalophilin in these fluids also has a smaller Stokes radius than that from other sources. On the basis of the isoelectric patterns and the Stokes radii the isoproteins of cobalophilin are tentatively divided into two populations, a glandular one occurring in milk and saliva and a myelogenic one occurring in all the cells and fluids studied but only occasionally in milk.